Polyaspartic acid manufacture

ABSTRACT

Polyaspartic acid is produced by hydrolysis of anhydropolyaspartic acid that has been produced by thermal condensation polymerization of L-aspartic acid. Conversion in excess of 80 percent is achievable utilizing &#34;temperature vs. time&#34; profiles.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of co-pending application U.S. Ser. No.08/080,562, filed Jun. 21, 1993, now U.S. Pat. No. 5,315,010, which, inturn, is a continuation-in-part of U.S. Ser. No. 07/660,355, filed Feb.22, 1991, now U.S. Pat. No. 5,221,733, granted Jun. 22, 1993.

FIELD OF THE INVENTION

The present invention relates to a method of polymerizing aspartic acidand to polysuccinimides and polyaspartic acids prepared by such method.

BACKGROUND OF THE INVENTION

Polyaspartic acids have utility as calcium carbonate and calciumphosphate inhibitors. Their biodegradability makes them particularlyvaluable from the point of view of environmental acceptability and wastedisposal.

Anhydropolyaspartic acids (i.e., polysuccinimides) are the anhydrousforms of polyaspartic acids.

Thermal condensation of aspartic acid to produce polyaspartic acid istaught by Etsuo Kokufuta, et al., "Temperature Effect on the MolecularWeight and the Optical Purity of Anhydropolyaspartic Acid Prepared byThermal Polycondensation," Bulletin of the Chemical Society Of Japan51(5):1555-1556 (1978). Kokufuta et al. teach that the molecular weightof the polyaspartic acid produced by this method increases withincreased reaction temperature. Moreover, the suggested maximum percentconversion of the aspartic acid to anhydropolyaspartic acid is no morethan 68% using oil bath temperatures of between 325° F. and 425° F.

A more recent work by Brenda J. Little et al., "Corrosion Inhibition ByThermal Polyaspartate" Surface Reactive Peptides and Polymers, pp263-279, American Chemistry Society Symposium Series 444(1990), citesKokufuta et al. Oil bath temperatures of 374° F. were reportedly used toproduce anhydropolyaspartic acid from powdered aspartic acid over aperiod of 24 to 96 hours. The reported results were no better than thosereported by Kokufuta et al., however.

SUMMARY OF THE INVENTION

The method of the present invention provides a much higher conversion ofL-aspartic acid to polysuccinimide and polyaspartic acid than has beentaught or suggested by the prior art. Moreover, contrary to theteachings of the prior art, the molecular weight of the polyasparticacid produced by our method does not increase with the reactiontemperature.

We have discovered that the thermal condensation of powdered L-asparticacid to produce polysuccinimide in relatively high yields optimallyoccurs above the initiation temperature of about 370° F., and preferablyoccurs above about 420° F., and most preferably occurs above about 440°F.

While a reactant temperature of less than about 370° F. may producepolysuccinimide over a period of many hours, the theoretical yields willbe low. The conversion of the L-aspartic acid to polysuccinimide islikely to be less than 70% over a period of many days.

On the other hand, as the reactant temperature is increased above 370°F., the percent conversion increases to greater than 90%, and thereaction times are greatly reduced.

The thermal condensation of L-aspartic acid to polysuccinimide accordingthe method of our invention produces a characteristically shaped"temperature vs. time" reaction curve. The curve is characterized by aninitial, rapid rise in reactant temperature, followed by an endothermsignaling the beginning of the reaction. Immediately following the onsetof the endotherm there is evaporative cooling, followed first by atemperature rise, and then by a second endotherm, which is followed byan evaporative cooling plateau. The temperature then rises to asubstantially constant plateau. The condensation reaction has gone to atleast 95% conversion at a temperature approximately midway between thefinal plateau and the time the temperature begins to rise to thatplateau.

Polyaspartic acid is produced from the polysuccinimide by basehydrolysis of the polysuccinimide.

The produced polyaspartic acid has a weight average molecular weight of1000 to 5000. This molecular weight range is uniform regardless of thepercent conversion.

The percent conversion of the L-aspartic acid to the polysuccinimide canbe increased in reduced time periods by increasing the temperaturesused.

Where the thermal fluid used to heat the L-aspartic acid is brought to500° F. in a reasonable time period, at least 90% conversion can beeffected within 4 hours.

Where the thermal fluid used to heat the L-aspartic acid is brought to amaintenance temperature of at least 550° F. within a reasonable timeperiod, at least 90% conversion can be effected within 2 hours.

Continuous as well as batch processes can be used. The process can becarried out in a fluidized bed; in a stirred reactor; in an indirectlyheated rotary drier, in an indirectly heated plate drier, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a temperature versus time reaction curve. Series 2 is theoil temperature. Series 1 is the reaction mixture temperature.

FIG. 2 depicts a temperature versus time reaction curve. Series 2 is theoil temperature. Series 1 is the reaction mixture temperature.

FIG. 3 depicts a temperature versus time reaction curve. Series 2 is theoil temperature. Series 1 is the reaction mixture temperature.

FIG. 4 depicts a temperature versus time reaction curve. Series 2 is theoil temperature. Series 1 is the reaction temperature.

FIG. 5 depicts a temperature versus time reaction curve. Series 2 is theoil temperature. Series 1 is the reaction temperature.

FIG. 6 depicts a temperature versus time reaction curve. Series 2 is theoil temperature. Series 1 is the reaction mixture temperature.

FIG. 7 depicts a temperature versus time reaction curve. Series 2 is theoil temperature. Series 1 is the reaction mixture temperature.

DESCRIPTION OF PREFERRED EMBODIMENTS

A series of experiments were conducted to thermally polymerize solidphase L-aspartic acid to polysuccinimide. In each instance, the powderedL-aspartic acid was added to a reaction vessel and heated. Samples weretaken throughout the course of the polymerization reaction. Thosesamples were analyzed for percent conversion to the product,polysuccinimide. The color and temperature of the samples were noted aswell. The produced polysuccinimide was then hydrolyzed to producepolyaspartic acid. Activity tests were conducted on the polyasparticacid.

Each of these, conversion, color, production of polyaspartic acid, andactivity are described below.

The following procedure was utilized to determine the percent conversionof the L-aspartic acid to the product, polysuccinimide:

THE DETERMINATION OF CONVERSION 0F L-ASPARTIC ACID TO POLYSUCCINIMIDE

A specific amount of the reaction mixture or product was dissolved in analiquot of dimethylformamide (DMF). The dissolution was allowed toproceed for 4 to 5 hours until all of the polysuccinimide dissolved inthe DMF, leaving unreacted L-aspartic acid which was filtered out. Theamount of unreacted L-aspartic acid was determined and used in thefollowing formula: ##EQU1## Where: A=weight of initial sample

B=weight of residue (unreacted L-aspartic acid)

COLOR

The color of each product sample was noted. The color of L-aspartic acidis white. The samples containing polysuccinimide varied in coloraccording to the temperature of the sample taken from the reactionmixture. From low temperature to high, the colors varied as follows:light pink, to pink, to tannish pink, to tan, to light yellow, toyellow, respectively. These colors generally corresponded to the percentconversion of the L-aspartic acid, in the same order with light pinkindicating the lowest percent conversion and yellow indicating thehighest percent conversion. The pink colors had less than 70 conversion.The literature has never reported any other color but pink.

POLYASPARTIC ACID

Polyaspartic acid was produced from polysuccinimide using the followinghydrolysis procedure:

Hydrolysis Procedure for Making Polyaspartic Acid from Polysuccinimide

A slurry was made from a measured amount of polysuccinimide anddistilled water. Sodium hydroxide was added dropwise to hydrolyzepolysuccinimide to polyaspartic acid. Complete hydrolysis was attainedat pH 9.5.

Bases other than sodium hydroxide can be used for hydrolysis. Suitablebases include ammonium hydroxide, potassium hydroxide, and otheralkaline and alkaline earth hydroxides.

Generally, the base is added to the polysuccinimide slurry until the pHvalue thereof reaches about 9.5, and a clear solution has been formed.

ACTIVITY TEST

Polyaspartic acid was produced from the samples of polysuccinimide. Theactivity of the polyaspartic acid as an inhibitor for preventing theprecipitation of calcium carbonate was determined as described in thetest below:

A standard volume of distilled water was pipetted into a beaker.Inhibitor (polyaspartic acid) was added after the addition of a calciumchloride solution, but prior to the addition of a solution of sodiumbicarbonate. Sodium hydroxide was then added to the solution until therewas an apparent and sudden calcium carbonate precipitation evidenced bythe cloudiness of the solution.

At this point the pH dropped, the addition of the sodium hydroxide wasstopped, and the pH was recorded. The volume of sodium hydroxideconsumed was noted. The pH drop after ten minutes was recorded.

The amount of inhibitor used was adjusted to provide a constant weightof polyaspartic acid in each of the tests.

The activity of the inhibitor was judged by the volume of sodiumhydroxide consumed and by the pH drop. The greater the amount of sodiumhydroxide needed, the greater the activity of the product as aninhibitor. The smaller the pH drop, the greater the activity of theproduct as an inhibitor.

MOLECULAR WEIGHT DETERMINATION

Gel permeation chromatography was utilized to determine the molecularweights of the polyaspartic acid produced. The molecular weightdeterminations were made on the polysuccinimide that was hydrolyzedusing the hydrolysis procedure described herein.

Rohm & Haas 2000 Mw polyacrylic acid and Rohm & Haas 4500 Mw polyacrylicacid were utilized as standards. The molecular weights provided for thepolyaspartic acid produced according to this invention are based onthese standards unless otherwise noted, and are reported as weightaverage molecular weights,(Mw). This is because molecular weights basedon gel permeation chromatography can vary with the standards utilized.

It was found that the molecular weight for the polyaspartic acidproduced fell within the range of 1000 Mw to 5000 Mw, regardless ofpercent conversion.

The term polyaspartic acid used herein also includes salts ofpolyaspartic acid. Counterions for polyaspartate include cations such asNa⁺, K⁺, Mg⁺, Li⁺, Ca⁺⁺, Zn⁺⁺, Ba⁺⁺, Co⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, and NH4⁺.

Polysuccinimide is the imide form of polyaspartic acid and is also knownas anhydropolyaspartic acid.

Conversion is defined to be the degree to which L-aspartic acid hasformed polysuccinimide by thermal condensation.

Equilibrium temperature is defined to be the temperature of the productupon completion of the reaction.

EXPERIMENTS

Reported below are examples of the production of polysuccinimide andpolyaspartic acid.

Laboratory Experiment 1

A "time vs. temperature" plot of the following reaction is depicted inFIG. 1.

A 500-ml covered, stainless steel beaker charged with 400 grams ofpowdered L-aspartic acid was placed in an oil bath. The oil bath wasquickly heated to a 425° F. maintenance temperature. The sample wasstirred throughout the experiment.

At 40 minutes, the reaction began when the first endotherm was reached.The first endotherm of the reaction mixture peaked at 390° F. at an oiltemperature of 425° F. which was the maintenance temperature.

Evaporative cooling immediately followed this first endotherm. Waterloss was evidenced by the evolution of steam. The reaction mixturetemperature dropped to a low of 360° F. during this period. Followingthe temperature drop, the reaction mixture began to heat up. At 2.75hours, the reaction mixture attained a plateau temperature of 400° F. Atthe end of 6.88 hours, 42 percent conversion had been attained. Steamcoming from the system evidenced water loss throughout the entireendothermic reaction. Evaporative cooling still continued to take place.The experiment was concluded after seven hours.

Table 1 below provides data developed during this experiment. Sampleswere taken at the times indicated and analyzed for percent conversion topolysuccinimide.

The relative activity of polyaspartic acid produced from the productpolysuccinimide was determined by the activity test described above.Activity is reported in terms of pH drop (δpH) and milliliters (ml) ofsodium hydroxide, as described in the Activity test.

The color of the reaction mixture is provided. Color was observed tovary with product temperature.

                  TABLE 1                                                         ______________________________________                                        POLYMERIZATION      ACTIVITY TEST                                             Time,  Product, Oil,    Conv. NaOH,                                           hr.    °F.                                                                             °F.                                                                            %     ml     δpH                                                                          Color                               ______________________________________                                        0.0    250      270      0    0.95   1.47 LP                                  1.0    386      430      5    --     --   LP                                  1.7    385      425     13    1.75   0.56 P                                   3.4    401      425     26    1.75   0.56 P                                   5.0    400      424     27    1.75   0.56 P                                   6.9    400      425     42    1.80   0.57 P                                   ______________________________________                                    

The following definitions apply through out this writing:

LP=light pink

LY=light yellow

P=Pink

T=Tan

W=White

Y=Yellow

Conv.=Conversion

δpH=activity test pH drop

hr=hours

Laboratory Experiment 2

A "time vs. temperature" plot of the following reaction is depicted inFIG. 2.

A 500-ml covered, stainless steel beaker charged with 400 grams ofpowdered, L-aspartic acid was placed in an oil bath. The oil bath wasquickly heated to a 450° F. maintenance temperature. The sample wasstirred throughout the experiment.

At 30 minutes, the reaction began when the first endotherm was reached.The first endotherm of the reaction mixture peaked at 395° F. at an oiltemperature of 439° F.

Evaporative cooling immediately followed this first endotherm. Waterloss was evidenced by the evolution of steam. The reaction mixturetemperature dropped to a low of 390° F. during this period and the oiltemperature rose to the 450° F. maintenance temperature.

Following the temperature drop, the reaction mixture began to heat up.At 1.67 hours, a second endotherm occurred. At this endotherm, thereaction mixture temperature was 450° F. and the oil temperature was450° F. Steam coming from the system evidenced water loss.

Evaporative cooling continued to take place until the conclusion of thesecond endotherm. Water loss was evidenced by the evolution of steam. Atthe conclusion of this period, the reaction mixture was then heated upand maintained at an equilibrium temperature of 434° F.

Table 2 below provides data developed during this experiment. Sampleswere taken at the times indicated and analyzed for percent conversion topolysuccinimide.

The relative activity of polyaspartic acid produced from the productpolysuccinimide was determined by the activity test described above.Activity is reported in terms of pH drop (δpH) and milliliters (ml) ofsodium hydroxide, as described in the activity test.

The color of the reaction mixture is provided. Color was observed tovary with product temperature.

                  TABLE 2                                                         ______________________________________                                        POLYMERIZATION      ACTIVITY TEST                                             Time,  Product, Oil,    Conv. NaOH,                                           hr.    °F.                                                                             °F.                                                                            %     ml     δpH                                                                          Color                               ______________________________________                                        0.0    340      345      0    0.95   1.47 W                                   0.5    400      440     22    --     --   LP                                  1.1    396      451     23    1.75   0.59 LP                                  1.7    422      457     32    1.80   0.57 p                                   4.2    416      451     58    1.81   0.61 p                                   5.5    420      452     81    1.80   0.63 T                                   7.1    430      454     97    1.75   0.69 T                                   ______________________________________                                    

Laboratory Experiment 3

A "time vs. temperature" plot of the following reaction is depicted inFIG. 3.

A 500-ml covered, stainless steel beaker charged with 400 grams ofpowdered, L-aspartic acid was placed in an oil bath. The oil bath wasquickly heated to a 500° F. maintenance temperature. The reactionmixture was stirred throughout the experiment.

At 30 minutes, the reaction began when the first endotherm was reached.The first endotherm of the reaction mixture peaked at 405° F. at an oiltemperature of 465° F.

Evaporative cooling immediately followed the first endotherm. Water losswas evidenced by the evolution of steam. The reaction mixturetemperature dropped to a low of 390° F. during this period, and the oiltemperature rose to 490° F.

At 1.25 hours, a second endotherm occurred. At this second endotherm,the reaction mixture temperature was 438° F. and the oil temperature was495° F.

Evaporative cooling continued to take place until the conclusion of thesecond endotherm. Water loss was evidenced by the evolution of steam.The reaction mixture temperature dropped to a low of 432° F. during thisperiod and the oil temperature rose to 499° F.

A diminution in evaporative cooling was evidenced by a steady rise inreaction mixture temperature between approximately 2.65 hours and %1%,hours. At 3.17 hours a temperature plateau was attained. No furtherincrease in conversion was noted beyond that point.

Table 3 below provides data developed during this experiment. Sampleswere taken at the times indicated and analyzed for percent conversion topolysuccinimide.

The relative activity of polyaspartic acid produced from the productpolysuccinimide was determined by the activity test described above.Activity is reported in terms of pH drop (δpH) and milliliters (ml) ofsodium hydroxide, as described in the activity test.

The color of the reaction mixture is provided. Color was observed tovary with product temperature.

                  TABLE 3                                                         ______________________________________                                        POLYMERIZATION      ACTIVITY TEST                                             Time,  Product, Oil,    Conv. NaOH,                                           hr.    °F.                                                                             °F.                                                                            %     ml     δpH                                                                          Color                               ______________________________________                                        0.0    256      316      0    0.95   1.47 W                                   0.5    406      464      7    --     --   LP                                  1.3    437      496     43    1.80   0.56 P                                   2.3    438      497     81    1.80   0.56 P                                   3.1    470      499     90    1.80   0.67 TP                                  3.8    476      500     95    1.80   0.63 TP                                  6.0    476      502     98    1.80   0.63 LY                                  ______________________________________                                    

Laboratory Experiment 4

A "time vs. temperature" plot of the following reaction is depicted inFIG. 4.

A 500-ml covered, stainless steel beaker charged with 400 grams ofpowdered, L-aspartic acid was placed in an oil bath. The oil bath wasquickly heated to a 550° F. maintenance temperature. The sample wasstirred throughout the experiment.

At 24 minutes, the reaction began when the first endotherm was reached.The first endotherm of the reaction mixture peaked at 410° F. at an oiltemperature of 470° F.

Evaporative cooling immediately followed the first endotherm. Water losswas evidenced by the evolution of steam. The reaction mixturetemperature dropped to a low of 395° F. during this period.

A second endotherm occurred at 1 hour at a reaction mixture temperatureof 442° F.

Evaporative cooling continued to take place until the conclusion of thesecond endotherm. The reaction mixture temperature dropped to a low of440° F. during this period.

A diminution in evaporative cooling was evidenced by a steady rise inreaction mixture temperature between approximately 1.5 hours and 2.06hours. At 2.06 hours a temperature plateau was attained. No furtherincrease in percent conversion was noted beyond 1.95 hours.

Table 4 below provides data developed during this experiment. Sampleswere taken at the times indicated and analyzed for percent conversion topolysuccinimide.

The relative activity of polyaspartic acid produced from the productpolysuccinimide was determined by the activity test described above.Activity is reported in terms of pH drop (δpH) and milliliters (ml) ofsodium hydroxide, as described in the activity test.

The color of the reaction mixture is provided. Color was observed tovary with product temperature.

                  TABLE 4                                                         ______________________________________                                        POLYMERIZATION      ACTIVITY TEST                                             Time,  Product, Oil,    Conv. NaOH,                                           hr.    °F.                                                                             °F.                                                                            %     ml     δpH                                                                          Color                               ______________________________________                                        0.0    330      348      0    0.95   1.47 W                                   0.5    405      470     11    --     --   LP                                  1.0    436      520     36    1.80   0.60 LP                                  1.4    439      536     66    1.80   0.67 p                                   1.8    462      540     92    1.80   0.58 TP                                  2.0    495      544     94    1.75   0.64 TP                                  2.4    510      547     96    1.75   0.58 LY                                  3.4    512      548     98    1.80   0.63 Y                                   ______________________________________                                    

Production scale product runs were conducted as follows:

Pilot Plant Test Run #1

A "time vs. temperature" plot of the following reaction is depicted inFIG. 5.

A DVT-130 drier-mixer manufactured by Littleford Brothers, Inc., ofFlorence, Ky. was used. The jacketed drier utilizes oil as a thermalfluid and a plough blade impeller. The drier-mixer had a stack open tothe atmosphere and a heat transfer area of 10 ft². The reactor's oilreservoir was preheated to 550° F. to provide an oil inlet temperatureof about 500° F.

The reactor was charged with 110.4 lb of powdered, L-aspartic acid. Hotoil began to flow through the jacket, and the impeller speed was set at155 rpm. Both the product and oil temperatures rose steadily. At aproduct temperature of 390° F., there was a sudden, endothermic reactionwhich caused the product temperature to drop (see FIG. 5). Water losswas evidenced by the evolution of steam. A sample taken revealed thatthe powder had changed from white to pink. Three percent of the materialwas converted to polysuccinimide.

Thereafter, product temperature began to rise steadily until it reacheda plateau at 428° F. which continued for an hour. Throughout this wholereaction, steam evolved, and the conversion increased in a linearfashion. At the end of the hour, the product temperature rose to 447° F.at which time the reaction underwent a second endotherm. Immediatelyafter this endotherm, steam ceased to evolve. Shortly after this point,the reaction was at least 88% complete. Following the second endotherm,the product slowly changed from a pink to a yellow color. The finalconversion was measured at 97%. Table 5 below provides data developedduring this experiment. Samples were taken at the times indicated andanalyzed for percent conversion to polysuccinimide.

                  TABLE 5                                                         ______________________________________                                        POLYMERIZATION                                                                Time, hr.                                                                              Product, °F.                                                                          Oil, °F.                                                                        Conv. %                                      ______________________________________                                        0.0       70            375       0                                           0.8      390            394       3                                           1.1      396            504      15                                           1.5      423            501      24                                           2.0      430            500      41                                           2.6      430            506      61                                           3.6      444            505      84                                           4.5      471            508      88                                           5.8      466            506      97                                           ______________________________________                                    

Pilot Plant Test Run #2

A "time vs. temperature" plot of the following reaction is depicted inFIG. 6.

A Littleford DVT-130 drier-mixer with a heat transfer area of 10 ft²,was charged with 110.4 lb of powdered, L-aspartic acid, and the oilreservoir was preheated to 525° F.

At the start up, hot oil began to flow through the jacket, and theimpeller speed was set at 155 rpm. Both the product and oil temperaturesrose steadily. The product temperature rose to 393° F. whereupon asudden, endothermic reaction caused the product temperature to drop (seeFIG. 6) and steam began to evolve. A sample taken revealed that thepowder had changed from white to pink. Four percent of the material wasconverted to polysuccinimide. Thereafter, product temperature began torise steadily until it reached a plateau at 427° F. which continued forone and a half hours. Throughout this whole reaction, steam was evolved,and the conversion increased in a linear fashion. At the end of thistime, the product temperature rose to 444° F. until the reactionunderwent a second endotherm. Immediately after this second endotherm,steam ceased to evolve. Shortly after this point, the reaction was atleast 94% complete. Following the second endotherm, the product slowlychanged from a pink to a yellow color. The final conversion was measuredat 98%. Table 6 below provides data developed during this experiment.Samples were taken at the times indicated and analyzed for percentconversion to polysuccinimide.

                  TABLE 6                                                         ______________________________________                                        POLYMERIZATION                                                                Time, hr.                                                                              Product, °F.                                                                          Oil, °F.                                                                        Conv. %                                      ______________________________________                                        0.0       70            400       0                                           1.0      393            488       5                                           1.3      400            476      18                                           2.0      428            475      20                                           3.9      441            480      66                                           4.4      450            477      85                                           5.1      456            476      94                                           6.1      457            484      98                                           ______________________________________                                    

Pilot Plant Test Run #3

A "time vs. temperature" plot of the following reaction is depicted inFIG. 7.

A "B" blender, manufactured by J. H. Day of Cincinnati, Ohio was chargedwith 110.4 lb of powdered, L-aspartic acid. The unit was a trough-shapedblender with a plough-bladed impeller and a heat transfer area ofapproximately 8 ft². The reactor was wrapped in fiberglass insulationbecause the oil heater was undersized. The reactor also had a largefunnel in a top port open to the atmosphere. The oil reservoir waspreheated to 500° F. At the start up, hot oil began to flow through thejacket, and the impeller began to rotate at 74 rpm. Both the product andoil temperatures rose steadily. The product temperature rose to 377° F.whereupon a sudden, endothermic reaction caused the product temperatureto drop (see FIG. 7) and steam began to evolve. A sample taken revealedthat the powder had changed from white to, pink. Thirteen percent of thematerial was converted to polysuccinimide. Thereafter, producttemperature began to rise steadily until it reached a plateau at 416° F.which continued for 3.75 hours. Throughout this whole reaction, steamwas evolved, and the conversion increased in a linear fashion. Due tothe heater being undersized, it took a longer time for the producttemperature to rise. At the end of this time, the product temperaturerose to 435° F. The reaction was at least 88% complete. Due to timelimitations, the reaction was stopped when the product temperaturereached the plateau. At this point, the final conversion was measured at90%. Table 7 below provides data developed during this experiment.Samples were taken at the times indicated and analyzed for percentconversion to polysuccinimide.

                  TABLE 7                                                         ______________________________________                                        POLYMERIZATION                                                                Time, hr.                                                                              Product, °F.                                                                          Oil, °F.                                                                        Conv. %                                      ______________________________________                                        0.0       55            390       0                                           1.0      370            420       0                                           2.3      377            448      13                                           3.0      403            455      21                                           3.5      416            460      26                                           4.0      417            469      32                                           4.5      416            471      38                                           5.0      416            472      45                                           5.5      415            460      52                                           6.8      413            446      64                                           7.3      414            448      70                                           7.8      418            451      74                                           8.3      422            455      81                                           9.3      433            460      88                                           9.8      435            460      90                                           ______________________________________                                    

These experiments show that degree of conversion of L-aspartic acid andthe time required for conversion are related to the temperature of thereaction mixture.

The higher the temperature of the thermal fluid used to heat thereaction mixture, the higher the degree of polymerization and the fasterthe rate of conversion.

Because of normal heat losses the temperature of the thermal fluid willalways be higher than the temperature of the reaction mixture. It isknown that increasing the temperature of the thermal fluid will increasethe driving force of the reaction. Assuming that the thermal fluidtemperature will be raised to its maintenance temperature in areasonably short period of time, we have found that generally thefollowing has held true:

Where the oil maintenance temperature was 425° F., at the end of 5 daysonly 60% conversion was achieved. The equilibrium temperature of thereaction mixture appeared to be 400° F.

Where the oil maintenance temperature was 450° F., 90% conversion tookplace within 7 hours. The equilibrium temperature of the reactionmixture is not known.

Where the oil maintenance temperature was 500° F., 90% conversion tookplace within 4 hours. The equilibrium temperature of the reactionmixture was 477° F.

Where the oil maintenance temperature was 550° F. 90% conversion tookplace within 2 hours. The equilibrium temperature of the reactionmixture was 510° F.

The difference between the maintenance temperature and the reactiontemperatures provides the driving force. Different means for providingthe thermal energy can result in different driving forces. Thus,although the relations derived here are qualitatively valid, there maybe some quantitative differences found in different systems. Differentthermal resistances will result in a shift in temperature and/or timerequirements.

The systems tested here tend to have high thermal resistance. Forsystems with less thermal resistance, lower source temperatures willsuffice to provide equivalent results.

The data indicates that continuous as well as batch processes can beused. The relationships discussed above are equally valid for both.Based on the data presented herein, a number of different reactors canbe used. Examples of these include, but are not limited to a heatedrotary drier; a thin-layer plate reactor; a stirred reactor; a fluidizedbed and the like. The reaction can be carried out at ambient pressure orunder a vacuum, as desired. The reaction can occur in air or a varietyof atmospheres, inert or otherwise. As a further example, an indirectlyheated rotary drier providing the same residence time as the DVT 130drier-mixer provides similar results under the same operatingconditions.

THERMAL CONDENSATION IN A STIRRED REACTOR

Apparatus: a hollow, cylindrical, stainless steel, jacketed vessel inthe form of a pan with a height of approximately 150 mm and a diameterof 400 mm. The vessel was fitted with four arms, each with severalattached plows. Thermal fluid was used to heat the vessel.

Procedure: The vessel was preheated to the desired temperature. A layerof L-aspartic acid was deposited in the pan and spread evenly across thepan. Samples were taken periodically to measure extent of reaction andconversion to polysuccinimide. Evolved steam was condensed. The observedresults are set forth in Table 8, below.

                                      TABLE 8                                     __________________________________________________________________________       Plate                   Layer                                              Test                                                                             Temp.                                                                             Sample                                                                            Conversion                                                                          Time                                                                              Plow  Depth                                              No.                                                                              (°F.)                                                                      No. (%)   (min)                                                                             Type.sup.1                                                                          (mm)                                                                              Comments                                       __________________________________________________________________________    1  500 1   33.88 20  Standard                                                                            13                                                        2   95.18 60                                                                  3   100.00                                                                              90                                                                            180                                                          2  400 1   1.8   40  Window                                                                              18                                                        2   8.4   70                                                                            80                                                           3  430 1   2.1   10  Window                                                                              18                                                        2   3.3   20                                                                  3   7.7   30                                                                  4   12.5  40                                                                  5   19.5  50                                                                  6   22.8  60                                                           4  500 1   27.5  10  Window                                                                              18  Cont. of No. 3                                        2   51.2  30                                                                  3   76.6  50                                                                  4   97.0  70                                                                            80                                                           5  430 1   .2    10  Window                                                                              17                                                 6  500 1   16.1  15  Window                                                                              18  Cont. of No. 5                                        2   28.9  30                                                           7  430 1   5.1   20  Window                                                                              18                                                                  20                                                           8  500 1   24.2  15  Window                                                                              18  Cont. of No. 7                                        2   39.5  30                                                           9  430 1   25.2  --  Standard                                                                            15                                                                  15                                                           10 500 1   37.3  --  Standard                                                                            15  Cont. of No. 9                                        2   66.9  --                                                                  3   91.0  --                                                                  4   99.6  --                                                                  5   99.5  --                                                                            110                                                          11 500 1   10.2  --  Serrated                                                                            15                                                        2   30.7  --                                                                  3   40.6  --                                                                  4   54.4  --                                                                  5   71.9  --                                                                  6   88.2  --                                                                            60                                                           12 455 1   6.7   --  Standard                                                                            15                                                        2   13.2  --                                                                            15                                                           13 464 1   8.1   --  Standard                                                                            15                                                        2   15.2  --                                                                            15                                                           14 500 1   18.3  --  Standard                                                                            15  Cont. of No. 13                                       2   43.3  --                                                                  3   73.9  --                                                                  4   95.9  --                                                                  5   99.2  --                                                                            60                                                           15 474 1   26.6  --  Standard                                                                            15                                                                  20                                                           16 500 1   52.3  --  Standard                                                                            15  Cont. of No. 15                                       2   85.3  --                                                                  3   98.6  --                                                                  4   99.4  --                                                                  5   99.3  --                                                                            75                                                           __________________________________________________________________________     .sup.1 In the above table, "standard" denotes a 90 mm solid plow; "window     indicates that the center portion of each plow has been cutout; and           "serrated" denotes a plow in which the bottom of the plow has teeth.     

THERMAL CONDENSATION ON A PLATE DRIER

Pilot plant scale tests were performed in a Krauss Maffei VTA 12/8 platedrier having eight stainless steel plates with a diameter of 1200 mm.The drier was operated in ambient atmosphere. The first two plates weremaintained at a relatively lower temperature than the next five. Thelast plate was not utilized due to temperature control problems. Eachplate has 0.35 m² of heat transfer surface and four arms of plows.Except for the first plate, the fourth arm of each plate had the plowsmoving in reverse in order to increase the retention time and theturnover rate.

L-aspartic acid powder was delumped and then fed to the first plate ofthe drier. The plows evenly distributed the deposited L-aspartic aciduntil thin ridges covered the plate. The deposited powder followed aspiral across the plate until it was pushed off the edge onto the nextplate. There the process was repeated until the powder was swept off thecenter of the plate. Once process conditions stabilized, samples weretaken from each plate to measure extent of reaction. The conversion ofL-aspartic acid to polysuccinimide could be followed by noting the colorchange of the powder by plate. The powder on the initial plates wasobserved to be light pink, on the middle plates a yellowish salmon, andon the final plates yellow-tan.

The observed results are compiled in Table 9, below.

                  TABLE 9                                                         ______________________________________                                             Plate            Air          Con-  Residence                            Test Temp.   Air Flow Temp. Plate  version                                                                             Time,.sup.2                          No.  (°F.)                                                                          (ft/min) (°F.)                                                                        No.    (%)   (min)                                ______________________________________                                        A    435     19       485   1      9.76  68.5/65.6                                                        2      12.38                                           480                    3      34.85                                                                  4      62.24                                                                  5      66.78                                                                  6      91.30                                                                  7      98.63                                                                  discharge                                         B    430     17       460   1      4.93  68.5/69.2                                                        2      16.44                                           495                    3      26.47                                                                  4      50.94                                                                  5      66.90                                                                  6      80.35                                                                  7      96.50                                                                  discharge                                                                            90.97.sup.3                                C    430     17       460   1            61.8/--                                                          2                                                      495                    3                                                                             4                                                                             5                                                                             6                                                                             7      98.72                                                                  discharge                                         D    440     17       462   1      19.71 61.8/42.1                                                        2      35.39                                           511                    3      39.01                                                                  4      89.52                                                                  5      99.01                                                                  6      99.03                                                                  7      99.40                                                                  discharge                                                                            99.67                                      ______________________________________                                         .sup.2 Total time spent in reactor/Calculated time based on 98%               conversion.                                                                   .sup.3 Includes bypass from feeder.                                      

Polysuccinimide produced in the foregoing manner can be readilyconverted to polyaspartic acid by base hydrolysis.

We claim:
 1. A method for producing polysuccinimide, comprising thesteps of(a) heating powdered L-aspartic acid to at least 370° F. toinitiate the condensation reaction and produce a reaction mixture, then(b) raising the reaction mixture temperature to at least 450° F., and(c) maintaining at least the 450° F. temperature for the reactionmixture until at least 80% conversion has occurred.
 2. The method ofclaim 1, wherein the reaction mixture temperature is raised to at least430° F. for a period of time sufficient to effect at least 90%conversion to polysuccinimide.
 3. The method of claim 1, wherein thereaction mixture temperature is raised to at least 440° F. andmaintained for a period of time sufficient to effect at least 95%conversion to polysuccinimide.
 4. The method of claim 1, wherein athermal fluid is used to heat the reaction mixture and the temperatureof the thermal fluid is raised to a maintenance temperature of at least500° F. and maintained at said temperature to effect at least a 90%conversion to polysuccinimide.
 5. The method of claim 4, wherein thethermal fluid is brought to a maintenance temperature of at least 550°F. to effect at least 90% conversion to polysuccinimide within about 2hours.
 6. The method of claim 1, further including the step of agitatingthe reaction mixture during the heating process for the purpose offacilitating heat transfer to the reactants.
 7. The method of claim 1further including the step of hydrolyzing the produced polysuccinimidewith a base.